The barrier effect of roadside diffracting elements for standard noise calculations

By using elements with quarter-wavelength resonators of different depths alongside a road, noise can be diffracted upwards to create a zone of noise reduction behind these elements. Measurements at a short distance, up to 15 meters, have proven that a noise reduction up to 4 dB(A) can be achieved. The noise reduction at long distance, up to 600 meters, was calculated numerically by combining a finite element model (FEM) and a parabolic equation model (PE), taking a downwind condition into account. These results are used to design an engineering approach that represents a barrier effect. This approach can be implemented in the standard noise calculation models, such as the Dutch national model or the ISO 9613-2 standard. The steps that are used for this engineering approach are explained. This involves the measurement of the intrinsic characteristics of the diffracting elements and empirical relations that use these characteristics.